The present invention relates to a printed coil unit for an actuator of a very small size and thickness.
With the term "actuator", it is intended to mean an apparatus composed of coils, magnetic circuits and the like for converting electric energy into mechanical energy through the medium of electromagnetic energy. The part constituted by the coils of the actuator is generally referred to as the coil unit. In the case of a coil unit where printed coils are used, most of the coil units are usually constituted by a laminated structure including a plurality of laminated or stacked printed coil sheets.
In accompaniment to remarkable technological progress in the field of audio, video, office automation (OA) and the like systems as well as increasing tendency to implement associated devices and equipment in a reduced size and thin structure, there arises a great demand for miniaturization and implementation of thin cross-sectional structure for the actuators employed in these systems or equipment.
Taking a flat brushless motor as an example of the actuator intended for the use mentioned above and referring to FIG. 2A of the accompanying drawings, the motor includes a magnet 21 uniformly magnetized divisionally in the circumferential direction to thereby constitute a rotor, and a coil unit 23 comprising a laminated structure of printed coil sheets (only one sheet shown for clarity) provided with spiral conductor patterns in a number corresponding to that of the magnetic poles of the rotor magnet, the coil unit 23 constituting a stator of the motor. Since the printed coil is very thin when compared with a wound coil, the actuator employing the printed coil unit permits the gap between a yoke plate 24 and the magnet 21 to be significantly reduced, as a result of which torque of an increased magnitude can be produced because of the increased intensity of the magnetic field at the location of the coil.
In an actuator of this type, a current supplied to the coils is switched by means of a current switching transistor which is controlled on the basis of an electric signal produced by a magneto-electric transducer element or magnetic sensor e.g., sensor element 22 in FIG. 2A element (or magnetism detecting element), such as a Hall element or a magneto-resistance element for detecting the rotational or angular position of the rotor magnet, the signal having a magnitude proportional to the detected field intensity. In this connection, it is noted that the very thin structure of the coil unit including the laminated printed coil sheets, which allows the distance or gap between the magnet 21 and the yoke plate 24 to be reduced can cause problems in installing the magnetic sensor element which occupies a relatively increased space in the gap between the magnet and the yoke plate. As examples of relatively thick components heretofore used for the magnetic sensor element 22, there may be mentioned a Hall element, a magneto-resistance element or the like all of which have thicknesses greater than 0.9 mm. Heretofore, the magnetic sensor element 22 has been mounted on the coil unit 23 with the gap being increased so that the sensor element is not brought into contact with the magnet 21 disposed in opposition thereto, as shown in FIG. 2B. With such disposition of the magnetic sensor element, it is however impossible to take advantage of the characteristic of the printed coil realize a very thin structure, because the gap distance between the magnet and the coil is increased due to the disposition of the sensor element. An attempt to dispose the magnetic sensor element at a position located outside of the main magnetic flux field of the magnet with a view to reducing the thickness of the actuator will encounter another difficulty in that the sensitivity of the magnetic sensor element is lowered, resulting in the need for an amplifier having high sensitivity to process the output signal of the sensor element. However, the signal-to-noise (S/N) ratio will be then degraded, and the overall expense of the actuator increased.
In the prior art actuator under consideration, there is also disposed between the printed coil unit and the magnet a frequency-generation (FG) coil formed in a unique substrate for detecting the moving speed of a movable part or member of the actuator. This further structural requirement generally involves increasing the thickness of the assembly as a whole. Japanese Utility Model Application Laid-Open No. 58-115853 discloses in more detail such a printed drive coil and a FB coil provided on the same carrier.